Joel Mudri, Fred Chen

Purpose: Rod-cone dystrophy (RCD) is characterised by progressive centripetal photoreceptor loss in the macula. This study investigates the feasibility of serial total macu-lar volume (TMV) measurements as a biomarker of dis- ease progression in RCD.

Methods: This prospective, single-centred observational cohort study included patients with clinical diagnosis of RCD and excluded those with cystoid macular oedema, epiretinal membrane, macular holes or recent cataract surgery. Baseline examination included best-corrected visual acuity and 250×300 spectral domain optical coherence tomography (61 slices, manual segmentation correction) with the Heidelberg Spectralis. Imaging was repeated six monthly for two years minimum. Linear regression was applied to TMV over time to determine TMV progression rates and the median time for this to decline by the retest variability threshold (RVT,
0.16 mm3) was calculated.

Results: Of 132 RCD patients, 42 with two-year follow-up were analysed. Twenty-seven (64%) were excluded leaving 30 eyes of 15 subjects for TMV progression analysis. Five of 15 patients showed a negative trend in TMV ranging from -0.03 to -0.14 mm3/year. Time to reach RVT varied from 1.13 to 4.74 (median = 1.81) years. No patient had significant best-corrected visual acuity change (>10 letters) in the study period. TMV change was symmetrical in all except one patient, due to asymmetric gliosis resulting in localised retinal thickening.

Conclusion: Coexisting cystoid macular oedema, epiretinal membrane and macular holes often confounds TMV measurements and even after their exclusion, TMV decline was only detectable a third of the remaining patients. In selected cases, TMV can decline by RVT within two years.